Not only partial pressure but volume percent and other variables as well. If memory serves 50ppm has been a rule of thumb for minimum volume percent threshold. Although I do not believe there is a great deal of data to support this particular threshold specifically(which is obviously quite low), there is quite a voluminous literature on specific alloys and specific services that support it in general.
Just a WAG here but uhm thinkin if there is a concern for H2S you need to have a response consistent with hydrogen service. It also seems reasonable to me that it is incumbant upon the end user and their engineering to determine if its hydrogen service.
As always, anything to do with corrosion is very service specific and cannot be accurately commented on in the general.

Thanks Guys for all of the input. I will have to go back to my Project Engineer and make him find out all of the particulars on the contract. I just don't want to end up wrong on this.